When Does a Supernova Become a Supernova Remnant?

This week is the 20th anniversary of the discovery of Supernova 1987A (SN1987A), the most famous supernova of our time. We asked Dick McCray, one of the foremost authorities on the outburst and its aftermath, whether SN1987A has become a supernova remnant, or are we still observing a supernova.

"It's a bit like the question of whether Pluto should be called a planet, except that people won't feel as passionate about the matter," McCray responded, and then added, "Maybe it's because Walt Disney didn't create a floppy-eared dog named supernova."

McCray went on to explain that, compared to the dilemma faced by planetary astronomers, it is a relatively simple matter to decide when the transition occurs. First, measure the total energy coming from the explosion and how it declines with time. Second, track the radiation produced when shock waves generated by the rapidly expanding supernova debris slam into surrounding gas. If the light from the explosion dominates, the object is still a supernova. If the light from the collision dominates, it has become a supernova remnant.

Performing this cosmic test is a straightforward, though complex task, using an assortment of telescopes tuned to photons ranging from infrared to gamma-ray energies.

What makes SN1987A extra-special is that astronomers have actually been able to carry out these observations. The supernova was observed at a time when all this technology was available (the explosion occurred 160,000 years ago in a nearby galaxy but the light did not reach Earth until 1987), and it is much closer to Earth than any other supernova that has been observed in modern times.

Almost all of the energy (more than 99%) emitted from SN1987A was in the form of neutrinos produced when the central core of the massive star Sanduleak 202 collapsed to form a neutron star. (Although the neutrinos actually arrived at Earth before the optical light, they were not found until a few days later, when physicists examined their data upon learning of SN1987A.) The neutrino outburst was over in a dozen seconds, but it had triggered all that would follow.

Many of the neutrinos did not escape from the exploding star, but were absorbed by portions of the star outside the collapsed core, creating a titanic shock wave. This shock wave swept through the outer layers of the star, fusing lighter elements into heavier ones, some of them highly radioactive.

The brilliant optical display first detected by astronomers in Chile, Zimbabwe, Australia and New Zealand on February 23, 1987 was produced primarily by the decay of radioactive nuclei of nickel, cobalt and titanium. The energy produced by the nuclear fission products of these elements was absorbed in the inner debris of the supernova.

Pent up heat was gradually released in the form of optical and ultraviolet light as the debris expanded, and reached a maximum luminosity of about 250 million suns about 3 months after the first detection. As the radioactive elements decayed to more stable nuclei, the energy source for the debris declined steadily, and it is now only about twice as luminous as the sun.

In the meantime, a shock wave produced by the expanding debris has been rumbling through the space around the supernova toward an inevitable encounter with material from the star's past.

About ten million years ago Sanduleak 202 formed out of a dark, dense, cloud of dust and gas. Roughly a million years ago, the star lost most of its outer layers in a slowly moving stellar wind that formed a vast cloud of gas around it. About 20,000 years ago, a high-speed wind blowing off the hot surface of the star carved out a cavity in the cool gas cloud.

The intense ultraviolet light from the supernova illuminated the edge of this cavity to produce the bright ring seen by the Hubble Space Telescope. The radiation from this ring was the primary source of radiation from SN1987A from an age of about 4 years until the year 2000, when SN1987A turned 13.

At that time, the shock wave created by the debris began to crash into the inner edges of the cavity. Since then, optical and X-radiation from the shock-heated inner edge of the gaseous ring has become the predominant source of radiation from SN1987A. A supernova remnant has been born.